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IC 


8882 



Bureau of Mines Information Circular/1982 



Reliability of Computerized 
Mine-Monitoring Systems 



By Raymond M. Kacmar 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8882 



Reliability of Computerized 
Mine-Monitoring Systems 



By Raymond M. Kacmar 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



A* 



W 1 




..„ — aSK* 1 



This publication has been cataloged as follows: 



Kacmar, Raymond M 

Reliability of computerized mine-monitoring systems. 

(Information circular ; 8882) 

Supt. of Docs, no.: I 28.27:8882. 

1. Mine safety— Data processing. 2. Mine gases— Data processing. 
3. Mine fires— Data processing. 4. Reliability (Engineering). I. Unit- 
ed Stales. Bureau of Mines. II. Title. III. Series: Information circular 
(United States. Bureau of Mines) ; 8882^ 



Ttt3e5r*M 622s [622'.8'02854] 82-600074 AACR2 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Advantages and limitations of monitoring systems 3 

Need for reliable systems 4 

Definition of reliability terms A 

Reliability research program. 8 

Conclusion 10 

ILLUSTRATION 

1 . Failure rate as function of equipment age 6 



RELIABILITY OF COMPUTERIZED MINE-MONITORING SYSTEMS 

By Raymond M. Kacmar 1 



ABSTRACT 

This paper describes the Bureau of Mines research program on the 
reliability of computerized mine-monitoring systems. The basic concepts 
of computerized monitoring are introduced along with its advantages and 
limitations. Current Bureau projects covering mine-monitoring systems 
are described, and some of the major areas of concern that should be 
addressed by future projects are outlined. 



^Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 




INTRODUCTION 



Federal regulations (Code of Federal 
Regulations, Title 30, Part 75) require 
the environmental monitoring of the 
atmosphere of underground mines for meth- 
ane, oxygen deficiency, carbon dioxide, 
air quantity, and indications of incipi- 
ent fires. Conventionally, this monitor- 
ing is done with stain tubes, flame 
safety lamps, machine-mounted devices, 
thermal detectors, and various portable 
instruments. Except for machine-mounted 
methane monitors, which are operative 
only while the machines are powered, and 
thermal detectors along beltways, this 
monitoring is intermittent and may not 
give adequate indications of the condi- 
tions between instrument readings. What 
is needed to improve this situation is a 
mine-monitoring system that operates con- 
tinuously and reliably so that unsafe 
conditions can be detected and dealt with 
promptly. 

Current systems that are being mar- 
keted to fill this need can be generally 
categorized as computerized mine- 
monitoring systems. The hardware com- 
ponents of this type of system basically 
consist of transducers that monitor vari- 
ous parameters in the mine and associated 
components that process the data 
obtained. The number of transducers and 
the amount of processing performed vary 
from system to system according to the 
manufacturer and also the complexity of 
the mine in which the system is 
installed. The transducers most commonly 
used for environmental monitoring measure 
one or more of the following parameters: 
Methane, coal dust, carbon monoxide, air- 
flow, temperature, relative humidity, 
differential pressure between airways, 
oxygen, noise, smoke, hydrogen sulfide, 
and submicrometer particulates. 

In addition to these, transducers 
can be deployed for gathering production 
management data, providing equipment 
maintenance supervision, inventory con- 
trol, event recording, and numerous other 
management reporting and documentation 
purposes . 



Data from all the transducers previ- 
ously discussed must be converted to 
engineering units, checked and processed 
to determine the status of conditions in 
the mine, and then displayed as needed. 

The equipment normally required to 
perform this function consists of one or 
more computers and the associated periph- 
erals and input-output interfaces. In 
the basic system the output from a trans- 
ducer is converted to a format that 
enables a signal to be transmitted to a 
central computer station. There, a pro- 
cessor (or system of processors) tabu- 
lates the data, compares it with preset 
alarm conditions, displays results, logs 
the data for future reference, and per- 
forms other calculations and data 
management. 

It is also possible to expand the 
system. This can be done by using a con- 
cept known as distributed processing. In 
this type of system, computing power is 
distributed by adding other noncentral- 
ized processing stations in the mine, 
thereby reducing the computational load 
on the central station. As an example, a 
processing station could be located 
adjacent to a group of transducers. This 
local station would be able to process 
the data and display results, alarms, or 
other information as needed. It would 
then send these data to the central sta- 
tion only as requested, allowing the cen- 
tral station to perform more system 
management operations. 

For a computer system to operate, 
however, it must be programed. Software 
modules must be written for each of the 
functions the system must perform, and an 
operating system must be developed that 
will combine these modules with the hard- 
ware to provide a working unit. Some of 
the main software modules inherent to 
monitoring systems follows: Initialize, 
read transducer data, test alarm limits, 
respond to keyboard commands, control 
peripherals, and manage communication 
system. 



These modules can be written either 
in the assembly language for the individ- 
ual processors used or in a high-level 
language, which can offer more 
versatility. 

To complete the system, a trans- 
mission scheme must be defined. Two 
generic types of technology that are cur- 
rently in use are the tube bundle and 



telemetry approaches. The tube bundle 
system aspirates gas samples to analyzers 
at a central location, usually on the 
surface. The telemetry system places 
transducers near the actual mine condi- 
tions to be monitored, and the data 
obtained are then encoded and transmitted 
to a central location either on the sur- 
face or underground. 



ADVANTAGES AND LIMITATIONS OF MONITORING SYSTEMS 



The tube bundle system has some 
desirable features and has been used 
where response times were not critical 
and high-accuracy instruments were 
desired to detect small changes in gas 
concentrations. Because the instruments, 
pumps, and valving can be located exter- 
nal to the mine, they are easily access- 
ible for maintenance, repair, and cali- 
bration. This allows the system to be 
independent of the underground mine 
power. Additionally, since the equipment 
can be located in fresh air, there are no 
problems with permissibility in coal 
mines. However, several limitations make 
this system not generally useful for 
multipurpose environmental monitoring; 
for instance, the system cannot be used 
for monitoring relative humidity, certain 
absorptive gases, or the operating status 
of various kinds of equipment. Its pri- 
mary usage has been for spontaneous com- 
bustion and fire detection monitoring. 

Telemetry systems are useful when a 
large number of points are to be mon- 
itored, fast response is required, and 
distances are large. The underground 
configuration usually has outstations 
located in fresh air connected to the 
common communications line and powered 
from the distribution system. The out- 
stations contain power supplies which are 
hardwired to power remote transducers 
located in the intake airways. If the 
transducers are located in the returns or 
face areas, the power supplies and trans- 
ducers must be permissible, usually 
utilizing intrinsically safe technology. 
This reliance on mine power seriously 
compromises system integrity from both a 
reliability and a functional usefulness 



point of view. Mine power is notorious 
for its wide fluctuations in voltage 
level and for the presence of very large 
and unpredictable transients which lead 
to early equipment failure or cause 
equipment malfunctions. Also, mine power 
is turned off routinely on both a sched- 
uled and an unscheduled basis, rendering 
the system inoperable. Sometimes this 
may be when the system can provide its 
greatest benefits, such as when the mine 
is unmanned. 

The main advantages of a computer- 
ized mine-monitoring system (either tube 
bundle or telemetry) derive from the con- 
tinuous nature of the data received from 
the system. These data show the output 
of the various transducers at any partic- 
ular time of interest and are continu- 
ously reported, providing indications of 
the levels of oxygen, carbon monoxide, 
methane, airflow, and other mine condi- 
tions as needed. Diagnostic data can be 
recorded; for example, fan speed and 
pressure can be monitored, providing 
advance warning of fan failures. Advan- 
tages also include the easy storage and 
retrieval of the data and the availabil- 
ity of the data at a central location 
with the possibility of adding additional 
readout units wherever necessary. 

An item of concern when installing a 
computerized mine-monitoring system mea- 
suring environmental parameters would be 
the location of the transducers. To 
provide complete and accurate coverage of 
an area of interest, several transducers 
may be required. In the face area, these 
modules would necessarily be located in 
high-traffic areas and would therefore 



have to be moved frequently. Also, 
because of the way current regulations 
are written, these systems would not be 
able to replace a certified person making 
the measurement and, therefore, would at 
present only provide additional data as 
far as complying with the regulations was 
concerned. 

In some applications, however, where 
certain operating conditions prohibit the 



development of extra entries, continuous 
monitoring of carbon monoxide levels 
along the beltway could enable the 
operator to petition the Mine Safety 
and Health Administration (MSHA) for a 
variance to enable the beltway to be used 
as a fresh air intake. This would 
be possible because of the data 
obtained from a continuous-monitoring 
system. 



NEED FOR RELIABLE SYSTEMS 



The environmental parameters that 
are being dealt with in underground com- 
puterized monitoring all have a serious 
and immediate impact on life safety in 
the mine. By necessity then, monitoring 
systems must have a high degree of opera- 
tional integrity to provide the protec- 
tion for which they are intended. 

The need for having a reliable sys- 
tem can be demonstrated in two 
approaches, safety and cost. In regard 
to safety, for a computerized mine- 
monitoring system the potential for three 
basic failure modes exists: Failing to 
give an alarm, giving a false alarm, and 
providing inaccurate data. The first 
failure mode gives the operator a false 
sense of security by implying that all 
conditions in the mine are safe, when in 
reality, a potentially unsafe condition 
exists. The second type of failure mode, 
false alarms, can cause problems by ini- 
tiating remedial action that is not 
necessary. Repeated false alarms also 
erode confidence in the monitoring sys- 
tem. This reduced confidence can result 
in indecisiveness during real emergen- 
cies, with the possible result of the 



operator simply turning off the alarm 
with the attitude of "it's just another 
false alarm, why bother to check it out." 
The third failure mode, inaccurate data, 
is included in addition to the other 
failures because of the continuous nature 
of the data received from the mine- 
monitoring system. These continuous data 
can be plotted and then compared with 
past data. When inaccurate information 
can be unknowingly recorded, it makes the 
decision of whether current trends are 
normal or unsafe less dependable. 

With regard to cost, the operator 
gets major benefits from reduced costs in 
the following areas: There is more 
feedback on production activities (for 
example, by knowing in advance that an 
unsafe condition is forming, the operator 
can make corrective changes before a 
problem arises that would force the 
shutting down of equipment); service and 
maintenance costs of the system are 
reduced; less inventory is needed to 
maintain the system; and acceptance by 
the mining personnel reduces malfunctions 
related to user dissatisfaction. 



DEFINITION OF RELIABILITY TERMS 



For a computerized mine-monitoring 
system to be accepted by the mining 
industry and to be used to its fullest 
potential, it must be demonstrated that 
it can provide the proper data and that 
this information can then be used to make 



logical decisions about the conditions in 
the mine. The data received from a com- 
puterized mine-monitoring system can 
only be meaningful, however, if the 
method of obtaining the data is 
reliable. 



Using a formal definition, reliabil- 
ity is the probability the system will 
perform its intended functions dur- 
ing a specified time interval, under 
stated conditions. Reliability planning, 
therefore, has to account for the envi- 
ronment the equipment will be subjected 
to and also reflect the amount of time 
during which the equipment will be 
operated. 

For reliability analysis, there are 
three basic failure categories, as shown 
in figure 1:2 

Early failures (infant mortality) - 
where the failure is due to manufacturing 
defects that were undetected by quality 
control checks. Decreasing failure rate. 

Chance failures (random failures) - 
where a system that has not failed is as 
good as new and, hence, its failure be- 
havior during any period of service 
depends only on the length of that period 
and not on the system's past history. 
Constant failure rate. 

Wear-out failures - where a system 
fails owing to the wearing out of com- 
ponents. Increasing failure rate. 

The useful life of a system, then, 
can be defined as the period of time 
after infant mortality and before equip- 
ment components wear out. 

Early failures occur during the ini- 
tial phases of an equipment's life and 
are normally the result of substandard 
materials being used or a malfunction in 

o ... . 

■^R&M Division Directorate for Product 
Assurance, U.S. Army Aviation Systems 
Command. Pocket Handbook on Reliability. 
September 1975, p. 11. 



the manufacturing process. When these 
mistakes are not caught by quality con- 
trol inspections, an early failure is 
likely to result. Early failures can be 
eliminated by a "burn-in" period during 
which the equipment is operated at stress 
levels approximating or exceeding the 
intended actual operating conditions. 
The equipment is released for actual use 
only when it has successfully passed 
through the "burn-in" period, usually by 
experiencing a specified period of time 
"failure free." 

Chance failures are those failures 
that result from strictly random or 
chance causes. They cannot be eliminated 
by lengthy burn-in periods or by good 
preventive maintenance practices. When 
the equipment's specified operating con- 
ditions and design levels are exceeded 
owing to random events, a chance failure 
could occur. 

Wear-out failures occur at the end 
of the equipment's useful life and are 
the result of equipment deterioration due 
to age or use. For example, light bulbs 
will eventually wear out and fail regard- 
less of how well they are made. The only 
way to reduce wear-out failures is to 
replace or repair the deteriorating com- 
ponent before it fails. 

Figure 1 illustrates that during the 
useful life period the failure rate is 
constant. A constant failure rate is 
described by the exponential failure dis- 
tribution. Thus, the exponential failure 
model reflects the fact that the item 
must represent a mature design whose 
failure rate, in general, is primarily 
comprised of stress-related failures. 
This means that early failures have been 
minimized and wear out is not noticeable 
or is beyond the period of concern. 



UJ 

a: 

2 



! 

\ Burn- in 

\ period Useful life period 


Wear-out / 
period / 


\ ! 


Wear-out failures 


tony ranures i chance failures 

i 



OPERATING LIFE (AGE), T 

FIGURE 1. - Failure rate as function of equipment age. 



The magnitude of this failure rate is 

directly related to the stress-strength 

ratio of the item. The exponential model 

can be derived from the basic notions of 

probability. When a fixed number, N Q , of 

components are repeatedly tested, there 

will be, after a time t, N s components 

that survive the test and N f components 

that fail. The reliability or probabil- 
ity of survival is at any time t during 

the test 



R(t) = 



N, 



N r 



N Q (N s +N f ) * 



Since N s = N Q - N f , reliability can be 
written 



The hazard rate z(t) is defined as 
the ratio of the fractional failure rate 
to the fractional surviving quantity; 
that is, the number of the original 
population still operating at time t, or 
simply the conditional probability of 
failure. 



z(t) = 



f(t) f(t) 
R(t) " 



f(t) 



l-F(t) ! _ p f (t)dt 



for the exponential distribution 
f(t) = X e" x t 
z(t) = X. 



and 



R(t) = 



and 



No ~ N f 



dR 



N f 

- 1 ~ iT = l ~ F(t) 
"o 



-1 dN< 



dt " N Q dt~ " f(t) l 



where f(t) ( = the failure density func- 
tion (the probability 

that a failure will 
occur in the next time 

increment dt.) 



In general, it can be assumed that 
the hazard rate of electronic elements 
and systems remains constant over prac- 
tical intervals of time, and that 



z(t), = X 



Hence, 



i » 



a constant, 



represents the expected number of random 
failures per unit of operating time of 
the i +h element (the failure rate.) 
Thus, when a constant failure rate can be 
assumed 



z(t), = X, = 



f(t) 



R(t), 



f _ 



-dR(t) t 

dt 
R(t), 



Solving this differential equation 
for R(t)| gives the exponential distribu- 
tion function commonly used in reliabil- 
ity prediction: 3 



R(t): 



- e"V> 



where R(t) = 



the probability the item 
will operate without 
failure for the time 
period t (usually ex- 
pressed in hours) under 
stated conditions, 



e = 2.7182...., the base of 
the natural logarithms, 

and X = the equipment failure rate 
(usually expressed in 
failures per hour) and is 
a constant for any given 
set of stress, tempera- 
ture, and quality level 
conditions. It is deter- 
mined for parts and com- 
ponents from large-scale 
data collection and/or 
test programs. 

When appropriate values of X and t 
are inserted into the above expression, 
the probability of success (reliability) 
is obtained for that time period. 

As a specific example, let X = 0.05 
failure per hour and t - 1 hour; then 



R(t) = e~ xt 



= P -0.05( 1) 



= 0.951. 



In other words, there is a 95.1-percent 
chance that the equipment will operate 
successfully for 1 hour. 



The reciprocal of the failure rate X 
is defined as the mean time between fail- 
ures (MTBF): 

MTBF = 1/X. 

The MTBF is primarily a figure of merit 
by which one hardware item can be com- 
pared with another. 

To obtain the failure rate and, 
therefore, the MTBF, a method for esti- 
mating part failure rates is needed. The 
most direct approach involves the use of 
large-scale data collection efforts to 
obtain the relationships between engi- 
neering and reliability variables and to 
develop factors for adjusting the relia- 
bility to estimate field reliability when 
considering application conditions. 

Failure data obtained from field use 
of past systems are not always applicable 
to future concepts . Data obtained on a 
system used in one environment may not be 
applicable for a different environment, 
especially if the new environment sub- 
stantially exceeds the design capabili- 
ties. Thus, a fundamental limitation on 
reliability prediction is the ability to 
accumulate data of known validity for the 
new application. 

Once the failure rates for the vari- 
ous components have been established and 
the MTBF of the system has been deter- 
mined, it would then be possible to 
develop a maintenance and repair program 
to insure high system availability 
[A(t)]« This is the ability of the sys- 
tem under the combined aspects of its 
reliability and maintenance to perform 
its required function at a stated instant 
in time (t). The key factors influencing 
availability, therefore, are mean time 
between failures (MTBF) and the mean time 
to repair the failures (MTTR) . This 
relationship can be expressed as 4 



3 Reliability Analysis Center (Griffiss 
Air Force Base, N.Y.). Reliability De- 
sign Handbook No. RDH376. March 1976, 
pp. 19-21. 



A = 



MTBF 
MTBF+MTTR * 



4 Page 292 of work cited in footnote 3. 



For an example, assume the MTBF for a 
system is 100 hours and that the MTTR is 
0.5 hour. The availability of the system 
then would be 



A = 



100 



100+0.5 

= 0.995. 

Therefore, the system would be available 
to perform its required function 
99.5 percent of the time. 

Reliability generally differs from 
availability because reliability requires 
the continuation of the normal state over 
the whole interval (o,t); which means, 



no failures. However, a component can 
still contribute to the system availabil- 
ity A(t) if the component failed before 
time t, is then repaired, and is normal 
again at time t. 5 

If the hardware failure rates of 
critical components are unacceptably 
high, these short-life components can be 
replaced with more reliable parts. 
Procedures can also be implemented to 
provide the necessary spare parts and 
tools for quickly repairing critical com- 
ponents when they fail. Thus, by 
increasing the system MTBF and decreasing 
the MTTR, the equipment will be more 
readily available to perform its intended 
functions. 



RELIABILITY RESEARCH PROGRAM 



A systems approach to the reliabil- 
ity problem has been undertaken by the 
Bureau of Mines. Studies are being done 
to look at both the overall performance 
of the system and the interaction of the 
various subsystems. 

For the hardware subsystem, work is 
being conducted on developing performance 
specifications for transducers that will 
be used in mine-monitoring systems. 
Also, a test criterion has been developed 
for the acceptability of mine instru- 
mentation. 6 This test criterion includes 
reliability acceptance testing of mine 
instrumentation developed using the 
guidelines of Military Standard (MIL- 
STD)781 (Reliability Design Qualification 
and Production Acceptance Tests) and will 
be used to determine if equipment meets 
the manufacturer's specifications when 
operated in a mine environment. The 
appropriate parameters of the mine 
environment that will affect the instru- 
mentation have also been developed under 
this effort. 

5 Henley, E. J., and H. Kumamoto. Re- 
liability Engineering and Risk Assess- 
ment. Prentice-Hall, Inc., Englewood 
Cliffs, N.J., 1981, p. 180. 



For the software of a computerized 
mine-monitoring system, there is 
presently no mechanism for specifying 
qualities or characteristics, such as 
reliability, maintainability, or usabil- 
ity. The importance of these software 
qualities is recognized by the Bureau, 
and in-house work is currently being done 
to review the work performed by the mili- 
tary and industry to determine if their 
approaches to software reliability can 
be applied to mine-monitoring systems. 
These approaches basically consist of 
software reliability models and automated 
verification systems. 

In the area of data transmission, 
in-house and contract work is being per- 
formed to evaluate various aspects of 
data security and transmission reliabil- 
ity. These studies will help to provide 
a reliable link between the transducers 
and the computer processors. 

To evaluate systems that are cur- 
rently being marketed, a methodology 

6 Trelewicz, K. Environmental Test 
Criteria for the Acceptability of Mine 
Instrumentation. Dayton T. Brown, Inc., 
BuMines Contract J0 100040, June 1981. 



for identifying safety hazards inherent 
in underground monitoring equipment is 
being developed. This methodology con- 
sists of performing functional analysis, 
fault tree studies, hazard analysis, and 
parts count reliability predictions. The 
information obtained from this methodol- 
ogy will be used to obtain approximate 
mean time between failure data and to 
evaluate the hazards associated with the 
failures that might occur. 

Another area of study will include 
the development of a test criterion and a 
test fixture for mine-monitoring systems. 
This test fixture will enable the evalua- 
tion of a mine-monitoring system when 
exposed to various simulated mine condi- 
tions and will provide a means of deter- 
mining system characteristics such as 
response time and ability to handle 
multiple-alarm conditions. 

An important link between all these 
areas is a proper data base. Because of 
the recent emergence of computerized 
mine-monitoring systems , not much infor- 
mation has been gathered about system 
reliability or system failure rates and 
causes during operation in a mine 
environment. Therefore, the Bureau has 
initiated several demonstration projects 
to acquire first-hand information on the 
operation of these systems. Currently, 
three mine-monitoring systems are being 
demonstrated. A minewide evaluation of 
an intrinsically safe system will be per- 
formed at the Lucerne coal mine. At 
Black River, a limestone mine, a hybrid 
telemetry-tube bundle system will be 
evaluated. The Bruceton demonstration, 
also a hybrid telemetry-tube bundle sys- 
tem, will be evaluated for its air- 
quality-monitoring capabilities in sup- 
port of the other projects. Ideas and 
equipment will be tested here first 
before installation in the Lucerne and 
Black River projects. 

In-house work will then be performed 
to set up the necessary data bases 
to support the current and future 
Bureau reliability studies. For example, 
transducer failure rates, calibration 



accuracies, data transmission error 
rates, and hardware and software failure 
rates and causes will all be evaluated. 
This information will then be used in 
developing reliability prediction models 
and mean-time-between-f ailure data. 

Future areas of study will be 
devoted to developing reliability design 
guidelines and specifications for com- 
puterized mine-monitoring systems. This 
effort will enable manufacturers, mine 
operators, and regulatory agencies to 
apply proven reliability techniques and 
procedures to computerized monitoring 
equipment being used in a mining environ- 
ment. These in-house projects will be 
conducted with the support of the Reli- 
ability and Compatibility Division of the 
Rome Air Development Center, Griffiss Air 
Force Base, N.Y. 

The guidelines developed will be 
generic in part, providing the manu- 
facturers with flexibility in specifying 
system functional and alternative modes 
of operation and physical boundaries, 
such as dimensions, weight, capabilities 
of materials, and power sources. They 
will, however, be more specific in areas 
such as defining the environmental pro- 
file, recommended alarm rates, transducer 
location, and redundancy of coverage, and 
determining which conditions actually 
constitute product failures. Then, the 
guidelines will be definitive in supply- 
ing proven reliability models and tech- 
niques. These can be applied by the 
manufacturers to their equipment in all 
phases of the system's life cycle. The 
models will include reliability block 
diagrams, probability equations, part 
failure modeling, prediction techniques 
(based on failure rate data obtained from 
Bureau demonstration projects and mine 
operators' data), and system modeling 
concepts pertaining to reliability as it 
impacts personnel safety, mission suc- 
cess, and unscheduled maintenance. 

Finally, a model for developing re- 
liability growth will be established that 
will account for detecting and analyzing 
hardware and software failures, feedback 



10 



and redesign of problem areas, implemen- 
tation of corrective actions, and 
retesting. 

Therefore, to provide an effective 
design program, the Bureau must develop 



reliability models that account for all 
of the system's life cycle factors and 
the environmental conditions to be 
encountered, and also provide for proper 
maintenance and feedback systems to 
insure reliability. 



CONCLUSION 



Computerized mine-monitoring systems 
can be used to fill existing gaps in 
environmental monitoring methods and to 
provide enhanced production monitoring. 
However, for a computerized system to be 
accepted by the mining industry, it has 
to be demonstrated that such a system 



would be reliable and would provide an 
advantage over current monitoring methods 
in safety and/or cost. It is the 
Bureau's intention to provide the proper 
data bases and guidelines to assure the 
mining industry that computerized mine- 
monitoring systems can be relied upon. 



INT.-BU.OF MINES, PGH., PA 26105 






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